PURPOSE: To prevent the effect of disturbance and to measure vacuum degree accurately, by providing a tubular electrode for preventing fluctuation in potential, which has the potential that is insulated from a potential of a part attached to a service port of a vacuum device, around an electrode assembly.

CONSTITUTION: An attaching flange 1 of an ionization gage is attached to a service or a vacuum container through attaching holes 1a. A circular cylinder shaped electrode 9, which is used for preventing fluctuation in potential in an electrode assembly comprising a filament 2, grids 3W6, and a collector 8, is arranged between the attaching flange 1 and an annular shaped mechanical protecting frame 7. In this constitution, even though a potential E of the attaching flange 1 is fluctuated, the potential of each electrode can be kept unchanged, the effect of disturbance is prevented, and the vacuum degree can be accurately measured.